Rotary furnaces or hearth furnaces have been used up to today for the melting of aluminum, its alloys and scraps. Usually natural gas or heating oil (heavy or light) are used as fuels. The necessary oxygen for the fuel is provided in the form of air. Because of the design of these burner systems and the furnaces, there is always enough excess oxygen in the combustion space so that there is quick oxidation of the liquid aluminum. In the language of the forger, “oxidation loss”.
To eliminate these undesired oxidation processes, especially in the rotary furnaces, the surface of the liquid aluminum must be protected from oxidation by a covering salt provided especially for this purpose. In the currently known processes, covering salt to the extent of between 7 and 12% of the charge weight is ad mixed into the melt.
The salt melts at such high temperatures (>1,000° C.) and forms a protective layer on the aluminum bath. Subsequently the salt reacts with the oxygen and converts into a loose, partly still exothermic slag. This slag called “scrapings” is eliminated from the furnace and must be temporaily stored on a dump belonging to the plant.
This process has the disadvantage that when the slag is removed it is partly still reacting exothermically and thus presents safety and environmental risks for the surroundings. In addition a good deal of liquid aluminum is trapped in the slag and this can only be reclaimed with much effort. The amount of this can be as much as 80 wt. % of the slag itself.
With the use of this method it is also known that the high temperature of the exothermic salt causes the included aluminum to melt and oxidize further. This leads to additional oxidation loss inside the slag and thus to even more loss, since the amount of recoverable aluminum in the slag decreases. In addition it is known that in the disposal chain between 20 and 25% of the slag is not recyclable and thus is a substantial burden to the environment, requiring storage in a landfill for dangerous waste.
The most important disadvantage to this process is, however, the loss of aluminum by oxidation during melting. The oxidation of the liquid aluminum can be controlled to some extent by using covering salt in the rotary and hearth furnaces, but loss cannot be avoided in the melting process itself since in this case the oxygen leads to the oxidation of the melting aluminum.
The losses range, depending on the charged material, between 1 and 12 wt. %, where pure block material has the lowest loss value and sheet or scrap with a large surface have the highest losses.